Copper sensitizers



United States Patent COPPER SENSITIZERS Edward B. Saubestre, Elmhurst, N. Y., assignor to Sylvania Electric Products Inc., a corporation of Massachusetts No Drawing. Application December 3, 1956 Serial No. 625,616

3 Claims. 01. 117-213 My invention relates to processes in which metal ions are chemically reduced to metal and deposited on a suitable electrically non-conductive substrate.

As known to the art, in this type of process, a metallic substrate is inserted in a bath containing in aqueous solution a salt, such as a chloride or sulfate of a metal to be deposited, a reducing agent, such as a hypophosphite salt, and a suitable buffering or pH controlling agent, such as sodium acetate. In the ensuing reaction, the metal salt is decomposed, and the metal itself is deposited on the surface of the substrate. While the mechanism of this process is not fully understood, it is believed that the hypophosphite ion continuously reduces the dissolved metal salt to the metallic state on the surface of the substrate by means of a catalytic action involving the surface.

It is well known that cobalt, silver, nickel, and chromium can be deposited in this manner. Other metals, however, can also be so deposited. For example, in the copending patent application of Philip H. Eisenberg, Serial No. 561,670, filed January 26, 1956, and

assigned to the assignee of the present application, there is disclosed a process for the deposition of iron, and, in my copending application, Serial No. 625,615, filed December 3, 1956, there is disclosed a process for depositing copper.

This process is defined as chemical reduction plating 0r deposition to differentiate it from the conventional electroplating or electrolytic deposition techniques.

It has been proposed to adapt this reduction technique for use in the printed circuit art by depositing metal upon the surface of an electrically non-conductive or insulator substrate. In order to deposit metals in this manner, it has been found necessary to first treat or sensitize the substrate surface in such manner that the metal to be deposited will be selectively deposited on this surface and will not be deposited elsewhere.

In the present state of the art, the insulator is sensitized by immersion in a solution containing a chloride salt of a metal selected from the class composed of silver, gold, platinum and palladium, these metals being defined as catalytic metals.

I have discovered that while the chloride salts described above are effective sensitizers when such metals as nickel, cobalt, silver and chromium are to be subsequently deposited on a sensitized surface, these salts are totally ineffective as sensitizers when copper is to be subsequently deposited.

Accordingly, it is an object of my invention to sensitize a surface of an electrically non-conductive substrate in such a manner that copper ions can be deposited and chemically reduced to metal on the sensitized surface.

Another object is to utilize chemical reduction plating to deposit copper upon a sensitized surface of an electrically non-conductive substrate.

Still another object is to sensitize a surface of an electrically non-conductive substrate through the use of "ice the hydroxide complexes of gold, platinum or palladium.

These and other objects of my invention will either be explained or will become apparent hereinafter.

In my studies of sensitizers, I have learned that the sensitizing operation proceeds in the following manner. First, the sensitizer salt is sorbed on the surface to be sensitized. Secondly, when this surface is immersed in the chemical reduction plating solution, the salt is reduced to metal by the reducing agent in the plating solution. Thirdly, the metal coating thus formed acts as a catalyst for initiating the subsequent copper deposition.

As is well known to the art, the ease of reduction of metal cations to metal is often greatly dependent upon the nature of the anion present. When I discovered that the chloride salts of the catalytic metals would not sensitize in such manneras to permit subsequent electroless deposition of copper, I postulated that the chloride, and not the catalytic metal itself, was preventing successful copper deposition.

In further investigations I discovered that the chloride was not fully disassociated from the catalytic metal, but rather that a chloride complex as for example palladium hexachloride, platinum hexachloride, or aurate tetrachloride, was formed. 'Such complexes are relatively stable. When the metal -to be deposited is easily reducible from an ionized state to its metal, as is the case for silver, for example, 'the relative stability'of these complexes has no deleterious effect. However, copper is not as easily reduced; apparently the relative stability of the chloride complexes is such as to prevent reduction of copper ions to metallic copper.

In experimenting with salts of catalytic metals other than chlorides, I discovered that the hydroxide complexes of platinum, palladium and gold'can be used as sensitizers and further that insulator surfaces sensitized by these hydroxide complexes can be readily coated with copper. (Silver hydroxide, being insoluble, does not form such a complex and consequently, cannot be used as a sensitizer.)

My invention will now be described in detail with reference to the examples which follow.

Example I Platinum chloride was dissolved in a sodium hydroxide solution to produce a platinate hydroxide complex Parts/v01. Rochelle salt sol n 25 Copper sulphat 25 Formaldehyde (37% concentration) 10 The deposition and reduction process was then initiated and a well bonded copper coating was deposited on the sensitized surface of the substrate.

Example II The process of Example I was repeated using an aurate hydroxide complex (HAuo the gold content being about .003% by weight, Substantially the same results were obtained.

3 Example III The process of Example I was repeated using a palladium hydroxide complex (Pd(OH) the palladium content being about .2% by weight. Substantially the same results were obtained.

The percentage content by weight of gold, platinum and palladium can be increased as desired providing that the hydroxide solution in which the metal is dissolved contains at least the appropriate stoichiometric quantity 1 of hydroxide. An excess quantity of free hydroxide can be present in the sensitizer solution, its presence not being deleterious.

As indicated by the preceding examples, the percentage content by weight of the precious metals can be extremely low. I have found, for example, that when aurate hydroxide is used, the gold percentage content by weight can be as low as .001%.

In forming the hydroxide complex, hydroxide solutions of materials other than sodium can be used, as for plexes of gold (HAuO platinum (Pt(OH) and palladium (Pd(OH) and electrolessly depositing copper upon the substrate so treated.

2. A method for sensitizing an electrically non-conductive substrate which comprises the steps of immersing said substrate in an alkaline solution containing at least one component selected from the class consisting of the hydroxide complexes of gold (HAu(O platinum (Pt(OH) and palladium (Pd(OH) the hydroxide content of said solution being present in at least stoichiometric proportions; and electrolessly depositing copper upon the substrate so treated.

3. In a method for depositing copper upon an insulator substrate, the steps of immersing said substrate in an alkaline solution containing at least one component selected from the class consisting of the hydroxide complexes of gold (HAuO platinum (Pt(OI-I) and palladium (Pd(OH) the hydroxide content of said solution being present in at least stoichiometric proportions; removing said substratefrom said alkaline solution, and immersing said substrate in a chemical reduction copper plating bath whereby copper ions are reduced to metallic copper on said substrate.

References Cited in the file of this patent UNITED STATES PATENTS 2,278,722 Loiseleur Apr. 7, 1942 FOREIGN PATENTS 503,034 Great Britain Mar. 30, 1939 

1. A METHOD FOR SENSITIZING AN INSULATOR SUBSTRATE WHICH COMPRISES THE STEP OF IMMERSING SAID SUBSTRATE IN AN ALKALINE SOLUTION CONTAINING AT LEAST ONE COMPLEX SELECTED FROM THE CLASS CONSISTING OF THE HYDROXIDE COMPLEXES OF GOLD (HAUO3-2), PLATINUM (PT(OH)6-2) AND PALADIUM (PD(OH)6-2); AND ELECTROLESSLY DEPOSITING COPPER UPON THE SUBSTRATE SO TREATED. 